There is a serious need to provide either service monitoring of safety-critical components and structures or to optimize their usage, especially if the structures or components are fabricated of composite materials. This need is particularly critical in the field of structures and components utilized for aerospace, naval and railway applications and in the construction field (e.g. bridges, viaducts, oil ducts, offshore platforms, etc.).
In fact, in these applications, the diagnostics need is paralleled by a requirement for reliable, miniaturized, portable monitoring systems. Optical sensors, either attached to the component surface (the structure) to be monitored or embedded where possible, are particularly suitable for these applications. Each of the indicated applications can benefit from the small, durable, long-lived, electromagnetically-immune capabilities of optical fibers to implement the optical sensing function.
The measurement of the parameters of interest is accomplished by the analysis of an optical signal, travelling within the fiber, that is perturbed in either frequency, or amplitude, or phase, or polarization, because of the interaction between the sensor and the parameter to be measured. Such a fiber sensor can have very small dimensions, can be attached durably or embedded easily in most structures and in most cases can provide a clearly measured representation of the perturbation without itself changing that perturbation.
The utilization of optical sensors is of particular interest in the service diagnostics of fiber reinforced polymers frequently used in the aerospace field. In fact for such structures, the optical sensors may be embedded in the structure during the fabrication process.
In the present state of the art, a method of embedding optical fibers within composite structures has been made available (see ALENIA Italian patent application RM93A000253, filed in Rome on Apr. 22, 1993) that guarantees the structural integrity of the fiber both within the material and at the entrance and exit points.
That application describes a method for embedding optical fibers, or sensors based on optical fibers, into components made of composite materials, with the purpose of performing the diagnostics of mechanical, thermal and chemical parameters inside the component during and after the manufacturing process, and while the component is being used for the purposes for which it has been designed and fabricated.
This method is particularly, but not exclusively, suited for applications to sensors based on optical fibers embedded into composite materials used in aerospace structures fabricated through different processes.
An example of such applications is that of a laminate made of a carbon fiber impregnated with a polymeric matrix, obtained through an autoclave process of compaction and polymerization of different layers, according to an assigned cure cycle of temperature and pressure.
This method may also be applied for systems providing monitoring and control of physical parameters of composite structures, while performing laboratory tests, as well as in service.
The method of that application simplifies and improves the fabrication process of composite structures with embedded optical fibers.
The application of sensors based on optical fibers to the monitoring of composite structures requires the embedding of optical fibers inside the layer of material not yet polymerized. At the end of the fabrication process of a structure made of composite materials, the optical fiber must be connected to the optoelectronic instrumentation to perform the analysis of the optical signals travelling along the fiber. Such signals contain information related to mechanical, chemical and thermal parameters. The optical fiber cannot come out of the composite material component through the external edge of the component, due to the required edge trimming to which the component is subjected. In addition, the method for embedding the optical fiber is simple, easy to be applied during the fabrication process, and able to preserve the integrity of the optical fiber and the quality of the manufactured component.
Any type of optical fiber can be used with this method. Optical fibers have a good mechanical strength under tension or compression loads, but they show a certain degree of brittleness when they undergo bending, if the radius of curvature is small. If the optical fiber comes out of the component without any special provision, it is highly probable that the fiber will be broken at the points of entrance and exit.
The method described in the aforementioned application has proved to be effective in the fabrication of components made of composite materials.
Embedding a sensor based on optical fibers into a component made of polymeric-matrix composites is performed by placing the optical fiber between contiguous layers of reinforcing materials. The component then undergoes the polymerization process through a temperature and pressure cycle, as required by the polymeric matrix.
The optical fiber is placed in such a way as to go into and come out from the component, in order to be connected with the instruments which generate and analyze the optical signals travelling along the fiber. In the present state of the art, the entrance and exit of the optical fiber are at a free end of the fiber emerging from the edge of the component.
The method protects an optical fiber embedded into a component made of composite materials, at the points where the optical fiber goes into and cones out of the component, by means of a small disk of rubber bonded on the surface of the component. This method also uses supports attached to a structure made of such components, for the purposes of protecting, after the embedding phase, the parts of the Optical fiber which are external to the structure, and for linking connectors with the optical fiber.
This method allows the optical fiber to go into and come out of the composite material component through the surface of the component instead of through the perimetral edge, thus obviating the difficulties related to the trimming of the component with the optical fibers embedded therein.
The method also ensures that the optical fiber will have high strength at the entrance and exit points, which are placed at the surface of the component, as the rubber disk results in an elastic constraint for the optical fiber at the entrance and exit points, and effectively protects the portions of optical fiber which are external to the component and to the end connectors.
Furthermore the operations, which are required to finish the component, can be performed easily, while maintaining the integrity of the optical fiber.
According to the invention, the layers of pre-impregnated material are cut as specified in the design drawings and the layers are then placed and compacted to form two pre-plied layers of pre-preg. Rubber disks are placed on the surface, one disk at each of the entrance and exit points of the optical fiber. Then a layer of release film is applied on the surface and the whole is compacted by using a temporary vacuum bag.
The upper pre-plied layer is then perforated, where the rubber disks have been placed, by means of hypodermic needles, which needles are left in their positions. The optical fiber is inserted through the needles, the needles are extracted and the optical fiber is tightened.
After fixing of the ends of the optical fiber to the release film, the upper pre-plied layer is placed on the lower pre-plied layer and two pre-plied layers are compacted by using a temporary vacuum bag.
The final vacuum bag is prepared by placing the release film, the breathing weave and a bagging film on the final stack resulting from the upper pre-ply compacted with the lower pre-ply layer and the bagging film is sealed on the tool by means of a sealant. A vacuum valve is placed on the vacuum bag, and air is drawn from the bag.
The assembly resulting from the operations indicated above is placed in an autoclave or oven or the like, and the requisite polymerization pressure and temperature are applied. The vacuum bag is then opened and the component is trimmed along its edges.
Protection elements are placed on the rubber disks, connectors are fitted at the ends of the optical fiber and the connectors are secured in their locations on the protection elements.
To date, several kinds of optical sensors for different applications (e.g. measurement of temperatures, strains) have been developed, based on different mechanisms of interaction between the parameter to be measured and the sensor (E Udd, Springer Proceedings in Physics Vol. 44, "Optical fiber sensors", Paris, France, Sep. 18-20 1989, pages 392-399). One sensor type of particular interest, the fiber grating sensor, was developed to operate in a Bragg system (Meltz, et al. Distributed, Spatially Resolving Optical Fiber Strain Gauge, "U.S. Pat. Nos. 4,761,073 and 4,806,012 of Aug. 2 1988 and Feb. 21 1989).
The grating is permanently exposed in the central core waveguide of the optical fiber through a process causing a periodic spatial modulation in the index of refraction along the longitudinal axis of the fiber. This grating constitutes an optical filter for the light radiation entering the fiber.
The light in the passband of the fiber grating filter is reflected back toward the optical transmitter during the preferred implementation. The intensity of the modulation of the index of refraction in the grating and the very same length of the grating determine the characteristics of the optical filter in terms of efficiency of reflection and bandwidth, while the grating pitch fixes the position of the filtering passband in the band of wavelengths. As an example, we consider the case of a Bragg grating sensor, embedded in the structure, that is illuminated by a wideband optical radiation. Mechanical deformations of the structure such as an elongation or a contraction cause a variation of the grating pitch and average index of refraction and, consequently, a shift in the filter function of the optical grating filter. This means that the signal that is reflected by the sensor is shifted in wavelength. This shift is, typically, a linear function of the variation of the grating pitch determined by the structure strains in the location of the sensor, in accordance with the relation (W. W. Morey, G. Meltz and W. H. Glenn, Springer Proceedings in Physics, Vol. 44, "Optical Fiber Sensors", Paris, France, Sep.18-20, 1989, pages 526-531): EQU .DELTA..lambda..sub.B /.lambda..sub.B =(1-Pe).epsilon.
where .lambda..sub.B= 2n.LAMBDA.is the central wavelength band of the optical signal reflected by the grating (n is the index of refraction for the propagation in the optical fiber and .LAMBDA.is the grating pitch), and .DELTA..lambda..sub.B its variation, Pe is the photoelastic constant of the optical fiber and .epsilon.is the value of the longitudinal strains, measured in microstrain. The above expression assumes that the sensor is perfectly integrated with the structure and is capable of following its deformations. This enables the Bragg grating sensor to measure the structure strains at the location of the sensor.
The necessary measurement (required to be performed on the light in order to convert the changes of the affected sensor into strain or temperature changes) is the shift .DELTA..lambda..sub.B of the Bragg filter wavelength before, during and after the perturbation. When the wavelength shift measurement is accomplished with conventional laboratory instrumentation, some unavoidable limitations in terms of heavy transportation weight, large instrument dimensions, long instrument measurement times can inhibit the ability to conduct such measurements in the field of service or at remote locations.
Quite recently, opto-acoustic devices have been made available in an integrated optics format (see ALENIA Italian Patent Application RM 93A000422 filed in Rome on Jun. 25, 1993) that are capable of operating as tunable optical filters.
That application describes an opto-acoustic device with a narrow wavelength bandwidth for filtering of guided optical radiation propagating along an optical fiber, such a device being rapidly tunable in a large bandwidth, insensitive to the polarization state of the optical radiation, without moving parts, robust and small in dimensions (&lt;5cm.sup.3). The device consists of a planar optical circuit, integrated on the surface of a lithium niobate (LiNbO.sub.3) substrate in which, separately for each input optical polarization, co-linear acousto-optic interactions with Surface Acoustic Waves (SAW) of suitable frequency are exploited to obtain polarization conversions, and a polarization filtering system is used in order to obtain a monotone correlation between the SAW frequency and the central optical wavelength of a narrow bandwidth implementing the optical transmission function of the device.
The combination of different technologies for the fabrication of the planar optical circuit, i.e. Titanium diffusion and protonic exchange followed by a thermal annealing, allows the integration of original key components, polarization splitters and combiners and the polarization filters, that are necessary to the architecture of the circuit to allow the device to be insensitive to the polarization of the optical radiation that has to be analyzed.
Furthermore the acoustic waveguides, that are integrated with the optical circuit, allow optimization of the device efficiency and increase its maximum resolution.
The invention in that application can be classified in the technical field of planar optical devices realized in integrated configuration to be interfaced with optical fibers, and in the application fields of: (a) optical fiber sensors, as a tunable filter for the spectral analysis of the optical radiation propagating along the fiber, or as a discriminator to measure the variation of the spectral optical line vs. time; and (b) communications through optical fiber networks operating on multiple optical carriers, for carrier multiplexing or demultiplexing.
In particular the new device there described for the narrow bandwidth (.delta..lambda./.lambda.&gt;10.sup.-4 to 10.sup.-5) filtering of optical radiations guided along an optical fiber can quickly be tuned in a large bandwidth ((.lambda..sub.max -.lambda..sub.min)/.lambda..apprxeq.0.2 to 0.3), and can also operate in a large optical wavelength range (500 to 2500 nm). It is insensitive to the polarization of the optical radiation, without moving parts and small in dimensions.
The device is realized on a substrate of lithium niobate (LiNbO.sub.3), cut orthogonally to the "X" axis of the crystal, in which two monomode optical waveguides are formed by titanium diffusion at high temperature. Two monomode optical waveguides, formed by proton exchange followed by thermal annealing, are both coupled at the ends of the titanium diffusion waveguides. The annealing step is used to have a refraction index profile for TE polarization similar to the index profile obtained by the titanium diffusion process. The input and the output of the optical signal correspond to the end faces of the titanium diffused waveguides. Parts of the waveguides realized by means of the proton exchange process, are close to the waveguides in proximity of their end faces, for a length and with a separation allowing the complete coupling between the two waveguides for the TE polarization.
In the length of the titanium diffusion waveguides between the coupling zones with the proton exchange waveguides and the points, where the superposition begins between the acoustical waveguides and the optical waveguides, titanium diffusion waveguides have at their sides, for a length of a few millimeters, two regions where a process similar to that used for the fabrication of the proton exchange guides has been effected, in order to obtain a stop filtering aimed at suppressing the residual TE component of the optical radiation.
Along the two parts in which the acousto-optic interaction occurs titanium diffusion guides are parallel to the "Y" axis of the LiNbO.sub.3 crystal, and guides are delimited by regions, where acoustic speed is higher than inside the acoustical waveguides. The increase in acoustic speed is obtained by a deep diffusion at a high temperature of an amount of titanium which is twice as high as that used to fabricate the optical waveguides. Two identical SAW transducers are formed at the ends of each of the two acoustical waveguides. The device is completed by two acoustic absorbers at the ends of the two acoustic waveguides.
The optical circuit of this device includes two kinds of waveguides: titanium diffused waveguides capable of propagating both TE and TM modes, and post annealed proton exchange waveguides capable of guiding only TE mode.
Two passive directional couplers are provided between the titanium waveguides and the proton exchange waveguides, so that TE radiation can be coupled from the first type to the second type and vice-versa, while TM mode propagates undisturbed into titanium diffused waveguides. In this way a polarization splitter is fabricated, Which does not require control voltage. Furthermore, the mixed technology permits in the same process step the fabrication of the hybrid directional coupler mentioned above and of the polarization filtering components at the input and output of the interaction regions. In fact, for the optimal operation of the device with respect to sensitivity, the polarization mode at the input of the interaction regions must be as pure as possible, and the residual input polarization mode at the output that has not been transformed by the acousto-optic interaction must be completely cancelled. These filtering functions are implemented by the transitions between titanium diffusion and proton exchange guides for TE and by the regions surrounding the titanium diffusion guides.
The two acousto-optical interactions exploited in the device occur in the central part. These two co-linear interactions, between the two polarized optical waves and the surface acoustical waves, produce the energy exchange between the two polarization state of the optical radiation. As previously described, the acousto-optical interaction occurs when the sum of the wave vectors of the input photon and phonon is equal to the wave vector of the output photon. Two different acousto-optic coupling regions are included in the device because of the two different polarizations of the input optical wave. The following operations on the optical radiation are necessary for a correct working of the device:
the TE and TM components of the input optical radiation must be separated and sent to the corresponding interaction regions by a polarization beam splitter;
the polarization of the optical waves at the input and output of the interaction regions must be correctly filtered, in order to have the TE (or TM) polarized radiation filtered at the output of the interaction region, at the input of which the TM (or TE) polarized radiation is sent by the polarization beam splitter; and
the optical output of the interaction regions must be combined into the optical output of the device by a beam combiner.
By so doing, the transfer function I(.DELTA..lambda.) is the transmission function of the filter. The input and output parts of the device provide the separation, recombination and filtering of optical polarizations. In the input part the two polarizations are separated, filtered and sent to the inputs of the corresponding interaction regions included in the central part of the device. The TM-polarized radiation enters the first titanium diffusion waveguide in the interaction region, at the output of which a SAW transducer is placed, in order to launch the SAW in the opposite direction with respect to the propagation of the TM-polarized optical wave. The TE polarized radiation enters the second proton exchange waveguide in the interaction region at the input of which the other SAW transducer is placed, in order to launch the SAW in the same direction as the propagation of the TE-polarized optical wave. This separation of the two polarizations allows the device to be independent of the polarization of the incoming optical radiation. As previously described, the optical wave and the acoustical wave propagate coaxially through the interaction regions. The increase in the SAW propagation speed in the two surface areas surrounding an unperturbed LiNbO.sub.3 channel allows the fabrication of the acoustical waveguide. The SAW propagation speed increases at the deep diffusion of titanium, the amount of which is roughly twice as high as that which is necessary for fabricating the optical waveguide.
The acoustical waveguides eliminate the diffraction loss of the SAW, with consequent increase in the efficiency of the device and in the acousto-optical interaction length. An increased length for acousto-optic interaction results, in turn, in narrower filter resolution. The two acoustical absorbers are used to eliminate SAW reflections at the edge of the device.
Such devices can be used to measure the shift of the reflected wavelengths of light from the fiber Bragg grating sensor or analyzing the associated transmitted signal. In a word, the integrated optics version of the acoustic-tuned filter is capable of performing the same functions as the laboratory instrumentation while overcoming the limitations of weight, size and measurement speed.
In fact, the integrated optics spectrometer has very small dimensions (&lt;5cm.sup.3), and its weight is mainly due to packaging materials that provide significantly reduced volume in comparison with laboratory instruments. Its performance is less affected than similar laboratory instruments by the effects of environmental conditions in which a structure of interest operates, e.g. vibrations, temperature changes, acceleration, electromagnetic interferences, etc. As such, it is operable outside a laboratory and is capable of analyzing the spectrum of the reflected signal with a very rapid response and with a resolution that can be implemented from 0.5 to 5 nm. Further, it requires much less electrical power (&lt;10mW) compared with 10's of watts that laboratory instruments require.